The invention relates to a method for producing hyperpolarized sample material for use in an NMR (nuclear magnetic resonance) investigation, in particular for use in an in vivo MRI (magnetic resonance imaging) experiment.
NMR techniques may be applied to gather information about a sample or sample area in a gentle, non-destructive way; in particular, clinical investigations on living patients can be done non-invasively. However, NMR techniques are generally limited by low signal intensities.
One way to increase signal intensities is to apply hyperpolarization techniques. Here, nuclei in a sample are prepared with a polarization level higher than corresponding to the Boltzmann distribution at the sample's temperature, and the hyperpolarized nuclei undergo an NMR experiment.
An important hyperpolarization technique is dissolution DNP (dynamic nuclear polarization). In this technique, a polarizing agent (typically based on a radical) is irradiated with microwave radiation, and the electron spin polarization of the polarizing agent is transferred to suitable nuclei, such as 13C. 13C is widely used as a nucleus to be investigated via dissolution DNP, since its longitudinal relaxation time T1 is long enough to complete typical NMR experiments before the polarization has decayed. The DNP process takes place at a cryogenic temperature, and after the polarization transfer, the hyperpolarized nuclei are dissolved for the NMR experiment.
A drawback of dissolution DNP on 13C or comparable nuclei with long T1 times, however, are relatively long times to prepare a hyperpolarized sample. At typical conditions, with a magnetic field strength of about 3.35 T and at a temperature of about 1.5 K and using the most common trityl radical, polarization levels of about 30-35% may be obtained, but polarization times have been found to be on the order of one hour for 13C. This poses a severe limitation in clinical applications. Further, temperatures of about 1.5 K are difficult to obtain.
Trityl is a relatively expensive type of radical. It has been tried to replace the trityl radicals with cheaper radicals such as TEMPO; however this has led to considerably lower polarization levels in 13C in DNP experiments.
A. J. P. Linde [17] proposed to combine a DNP technique, hyperpolarizing protons, with a Cross Polarization technique, transferring the polarization from the protons to 13C. The DNP was done at a static magnetic field of 3.35 T and at a temperature of 1.5 K, and applied TEMPO radicals as polarizing agent. In Linde's experiments, polarization levels of 13C comparable to those he obtained in direct DNP on 13C with trityl radicals were obtained in a few minutes. In order to improve his technique, Linde proposed to lower the temperature of the system to 1.2 K.
It is the object of the invention to provide an method by means of which nuclei with a long longitudinal relaxation time T1 can be quickly hyperpolarized to a high polarization level.